Alloy steel



nite States Patent 2,797,991 Patented July 2, 195

ALLOY STEEL Charles W. Laufle, Detroit, and Clarence L. Altenburger,

Dearborn, Mich., assignors to National Steel Corporation, a corporation of Delaware N Drawing. Application November 22, 1954, Serial No. 470,534

1 Claim. (Cl. 75-123) This invention relates to improvements in low alloy steels. More particularly, the present invention relates to low alloy steels capable of being produced in the hot rolled, normalized, or cold reduced-annealed condition with high yield strengths.

Low alloy steel cold forming operations producing shapes and contours 'of varying degrees of curvature and deformation in light plate, sheet and strip steel, herein all termed sheet stock, generally employ steels with assured minimum yield strengths of 38,000 pounds per square inch upw'ard. It is desirable to provide a steel which is capable of being produced in the hot rolled, normalized, or cold reduced-annealed condition with physical properties which can be varied at will by con trolled changes in the chemical analysis to obtain a desired yield strength of 38,000 p. s. i. upward. Superimposed upon these requisite selective tensile properties are the requirements of toughness and ductility at normal and subnormal temperatures rendering the steel reliable under conditions of complex stress distributions combined with high velocity of application of stress and/ or low temperatures. A further characteristic required for such cold forming operations is la fine-grained metallurgical structure in the hot rolled, normalized, or cold reduced-annealed condition. Where a fine-grained structure is not present in the steel, cold forming of sheet stock causes a coarsened bumpy surface, commonly called orange peel, in those areas of the work piece which have been severely cold worked. The more severe the cold working the more pronounced is the so-called :orange peel condition. if such :a condition occurs, the stamping must be polished to a degree which will remove this undesirable surface appearance. Polishing, however, is a highly expensive procedure. It is therefore desirable to produce a steel which is capable of being subjected to severe cold working operations, such as the drawing operations necessary to form body panels, bumpers and bumper guards for automobiles and trucks, without requiring extensive polishing after forming in order to eliminate orange peel surface conditions.

Other surface disturbances which are undesirable in a finished stamping and which are especially intensified in that they are more noticeable on polished surfaces or chrome plated objects are stretcher strains, cross breaks and other variants of stretcher strains such as rollerleveler breaks, coil breaks and bend breaks. These surface conditions are highly undesirable in finished stampings which are exposed to view in use and particularly so in parts designed to be ornamental as well as functional, such as automobile pumpers, bumper guards and the like. The stretcher straining propensity of steel is commonly corrected by passing such steel through a cold rolling operation known as a temper pass in which the steel is elongated by cold rolling a given percentage of its length which is generally of the order of /z to 2 /2 percent. Such a treatment does alleviate stretcher strain tendency for a certain length of time. However, by a process known as strain aging, the tendency to stretcher strain gradually returns upon standing at room temperature. Such changes also involve increases in hardness, tensile strength and yield strength. This results in a loss of ductility and toughness. Strain aging is greatly accelerated by relatively small increases in temperature. Thus in the case of subjecting steel .of the type which has had stretcher strainability removed by a temper pass to high tempertaure operations such as flat polishing where the temperatures may be as high as 260 F., stretcher strainability rapidly returns and upon subjecting such steel to stamping operations, stretcher strains or similar surface conditions result. It would therefore be highly desirable to provide a steel which is immune to such strain aging even when subjected to high temperatures which accelerate strain aging and subsequently cause stretcher straining when the steel is cold formed.

Another absolutely necessary requirement of a steel of this nature is the ability to be welded to produce 'a satisfactory joint without the necessity of using special electrodes, techniques or precautions. Although the low alloy steels of the prior art have had satisfactory ability insofar as are or heliarc weldability is concerned, their spot projection and resistance weldability have generally left much to be desired in that special precautions must be taken to accomplish such weldments. Hence it is extremely important that spot projection and resistance weldability be improved in low alloy steels so that such special precautions may be reduced to a minimum.

Since the low all'oy steels of the type with which the present invention is concerned are often subjected to ta flat polishing operation prior to stamping, the ease with which the steel may be flat polished reflects the value of the steel. Thus it is arequisite of these steels that their physical properties be maintained at the highest value consistent with other requirements to reduce to a minimum the load-up propensity of .the labrasive belts in the flat polishing equipment. The ease of fiat polishing depends in good part upon the degree of imperfections in the surface as described above and including pits which are a result of scale rolled into the surface of the metal in the finishing operations. Since steels of this nature are generally temper passed with hot rolled scale present, it is desirable that the .hot rolled scale adhere firmly t0 the steel surface so as to lessen the degree of pitting from loose scale particles.

Accordingly, it is a principal object .of the present invention to provide an improved low alloy steel characterized by an assured yield strength of 38,000 pounds per square inch upward in the hot-rolled, normalized, or cold reduced-annealed condition.

It is a further object of the present invention to provide a steel of the aforesaid assured yield strength combined with toughness and ductility characteristics as required to withstand normal and subnormal temperatures under conditions of complex stress distributions combined with high velocity of application of stress.

A further object of the present invention is to provide a steel consistent with the foregoing objects and characterized by a fine-grained metallurgical structure when produced in the hot rolled, normalized or cold reducedannealed condition, such fine-grained structure serving to eliminate objectionable orange peel surface conditions.

A further object of the present invention is to provide a low alloy steel consistent with the foregoing objects and further characterized by stretcher strain immunity even when subjected to elevated temperature conditions.

Another object of the present invention is to provide a low alloy steel consistent with the foregoing objects and further characterized by improved spot projection and resistance weldability characteristics.

Another object of the present invention is to provide a low ialloy steel consistent with the foregoing objects and further characterized by high hot rolled scale tenacity on the surface of the steel permitting temper passing with a minimum degree of pitting.

These and 'other objects of the present invention will become more apparent after considering the following specific description of the same.

In accordance with the present invention we have discovered that alloy steels containing:

0.060.15% carbon 0.350.75% manganese 0.4 -0.9% silicon 0.050.15% zirconium and the remainder iron, together with incidental impurities, possess the foregoing enumerated requisite properties providing the percent silicon to percent manganese ratio is maintained within the range of 1.0 to 1.5, inclusive. It is preferred that the maximum phosphorus and sulfur content of these alloys be not more than 0.03% each.

Low alloy steels falling within the aforesaid composition and having a percent silicon to percent manganese ratio within the range 1.0 to 1.5, inclusive, have been found to have a fine-grain structure and can be produced with assured yield strengths of 38,000 p. s. i. upward in the hot rolled, normalized, or cold reduced annealed condition. An unexpected property is a uniform high level ductility at a given level of strength. Not only are the ductility levels relatively high at a given strength level, but they are relatively more uniform from heat to heat and within heats than are comparable prior art low alloy steels of the same yield strength. Accompanying the unexpectedly higher level of ductility at a given level of strength is a much greater freedom from loss of quality due to inclusions. Since inclusions generally lead to directional properties resulting from unavoidable variations in steel making practices, the present steels are superior from a uniformity standpoint.

The following are examples of steels in accordance with the present invention:

The yield strength data in rounded figures was obtained from hot rolled steel at 0.100 inch thickness obtained on a 34-inch mill with a 16801720 F finishing temperature and a piling temperature 'of 1200 F. or less. Piling temperatures below 1200 F. do not appear to cause significant variations in the physical properties. Piling temperatures above 1200 F., on the other hand, do cause variation downwardly in both yield and tensile strength. Other undesirable metallurgical properties result from high piling temperatures. 1

Olsen ductilities determined with a inch ball for each of the above examples were within the range 0.445 to 0.470 inch, as compared to other prior art low alloy steels having inch ball Olsen ductilities within the range 0.400 to 0.470 inch. The average of the Olsen ductilities of the foregoing examples is approximately 0.450 inch, whereas the average of the same prior art low all'oy steels is about 0.425 inch. Specific data illustrating uniform hardness from edge to edge of an as rolled sheet of indicated thickness whose chemical analysis was substantially the same as Example 3 above, is as follows:

The foregoing examples are for the purpose !of illustrating the present invention and are not limiting to the scope thereof which is set forth in the appended claim.

We claim:

A low alloy steel cold drawn sheet stock characterized by a fine-grain metallurgical structure, Ia high hot rolled scale tenacity, stretcher strain immunity, a uniform high level ductility and good spot projection and resistance weldability and producible in the hot rolled, normalized, or cold reduced-annealed condition with an assured yield strength of 38,000 p. s. i. upward, the composition of the low alloy steel consisting of 0.35-0.75 manganese, 0.40.9% silicon, 0.050.15% zirconium, from about '0.060.15% carbon, a maximum of 0.03% phosphorus, a

maximum of 0.03% sulfur, and the remainder iron together with incidental impurities, the ratio of silicon to 'm anganese being maintained in the range 1.0 to 1.5 inclusive.

References Cited in the file of this patent UNITED STATES PATENTS 1,954,297 Keir Apr. 10, 1934 2,046,168 Kinzel et al. June 30, 1936 2,474,766 Waggoner June 28, 1949 

